I know it only as an uncouth word for female genitals, an idiot/fool or as a verb, to hit someone.

I always believed that twat when used as a description of a pregnant fish was nothing more that urban myth, apparently this may not be so. There is also a theory that it can also be used as a synonym for a nun's wimple. Not in the dictionaries where I have looked.

Of twat as a vulgar term for the vulva, Michael Quinion says that it is "a low slang term dating from the seventeenth century, whose origin is unknown". For that word, my copy of Random House Webster's New Universal Unabridged Dictionary nevertheless hypothesizes a connection with the Anglo-Saxon thwat, "akin to the Old Norse thveit (cut slit, forest clearing)", and a parallel connection with the current northern English word thwaite (which appears as a suffix in many place names, e.g. Braithwaite, Crosthwaite, Micklethwaite), meaning 'forest clearing'.

As in your case, I found no mention of the 'pregnant fish' or 'wimple' connotations in any dictionary that I (by which I really mean OneLook) know about.

Incidentally, if one person tells another, "I can see your twat", the chances are they are not referring specifically to the vagina (which is an internal organ), but to their genitals in a general sense.

So it's surprising that not all dictionary compilers seem able to distinguish between a vagina and a vulva when they define twat.

For instance, Macmillan Dictionary says it is "an extremely offensive word for a woman's vagina". Cambridge Advanced Learner's Dictionary says, "offensive for the vagina". You'd think they would know better.

Then, owls and bats, cowls and twats,
Monks and nuns, in a cloister's moods,
Adjourn to the oak-stump pantry !

[After she has begun to undress herself.
Now, one thing I should like to really know :
How near I ever might approach all these
I only fancied being, this long day !
Approach, I mean, so as to touch them, so
As to ... in some way . . . move them if you please,
Do good or evil to them some slight way.
For instance, if I wind
Silk to-morrow, my silk may bind

Which is from Pippa Passes by Robert Browning published in 1841. Thee thinking is "monks wear cowls so nuns must wear...

"Twat" both then and now is vulgar slang for a woman's external genitals. It has become a relatively mild epithet in parts of the UK, but vulgar elsewhere. When the editors of the Oxford English Dictionary enquired decades later where Browning had picked up the word, he directed them to a rhyme from 1660 that went thus: "They talk’t of his having a Cardinall’s Hat/They’d send him as soon an Old Nun’s Twat." Browning apparently missed the vulgar joke and took "twat" to mean part of a nun's habit, pairing it in his poem with a priest's cowl.[2][3] The mistake was pointed out by H. W. Fay in 1888.[4]

Native speakers of British English might be surprised (as I was) to discover that here in the USA, in many parts of the country the usual pronunciation is 'twot'.

...and if you're visiting North America, and want to call someone an idiot or a fool, it'd be safer to use those words. Twat has only one meaning in my neck of the woods (and it's not the past tense of tweet). Threatening to twat someone in the face would likely cause more confusion than anger.

Various other sources accurately pointed out that a pregnant goldfish (among fishes, goldfish seem to be particularly prone to having the label twat attached to them) lays eggs rather than giving birth to live young, and hence the correct technical term is gravid, not pregnant.

Judging from the large number of hits I found which promote the notion that 'pregnant goldfish' are called twats, there is either some kind of systematic campaign to manufacture a new definition into being by bringing to bear the sheer brute force and unquestionable moral authority of the internet, or there are lots of gullible wishful thinkers who are rather tickled by the thought that a naughty word can also be applied to a seemingly more innocent entity. (Though it must be pointed out that once it is -- ahem -- 'pregnant', even the most demure-looking goldfish can hardly be regarded as a model of unbesmirched sexual propriety.)

Or possibly there is both a vast conspiracy mobilizing for the piscine type of twat and a vast army of credulous twits...